Liquid crystal displays, especially, liquid crystal displays having thin film transistors (TFT) have been widely used as displays in various applications from mobile phones to large-sized televisions.
One use is in a personal display device, the display screen of which is required to be seen by a user of the personal display device, but not to be seen by other persons who view the personal display device from the side thereof.
The personal display device can be constructed such that the display screen of the personal display device can be viewed by a large number of persons, or the display screen can be exclusively used by only one individual, as occasion demands.
FIG. 7 is a view illustrating a conventional liquid crystal display having a “secret” mode. (for example, Japanese Unexamined Publication No. 5-72529). A backlight for emitting light to a liquid crystal panel from the rear side has high directionality.
Between the common liquid crystal panel and the directional backlight another liquid crystal panel is disposed for switching between a scattered light state and an unscattered light state. This liquid crystal panel may be, for example, a polymer-type liquid crystal panel (a scattering-unscattering switching panel).
When the scattering-unscattering switching layer is in an unscattered state, light from the backlight is emitted to the front direction only, and therefore, it is not possible to see the display from the side.
When the scattering-unscattering switching layer is in a scattered state, on the other hand, light from the backlight is emitted in the inclined directions, and therefore, it is possible to see the display from a side thereof. Consequently, a large number of persons can view the display.
To provide the secret mode of operation, it is necessary to manufacture a special liquid crystal panel different from the common liquid crystal panel, and therefore, the manufacturing costs are increased.
In order to solve this problem, another method has been proposed using a vertical-alignment-type liquid crystal display. FIGS. 8A and 8B are views respectively illustrating the shape of a liquid crystal molecule when viewing the vertical alignment type liquid crystal display from the front.
In a state that the voltage is not applied, as shown in FIG. 8A, the liquid crystal molecule is aligned vertically. When the voltage is applied as shown in FIG. 8B, the liquid crystal molecule is inclined in an orthogonal state.
A polarizer and an analyzer are directed with their absorption axes in the vertical direction and in the horizontal direction, respectively.
FIG. 8A illustrates a state where the vertically aligned liquid crystal panel, to which the voltage in not applied, is viewed from the front. Double refraction of the liquid crystal molecule does not occur, and light does not leak.
FIG. 8B illustrates a state where the vertically aligned liquid crystal panel, to which the voltage in applied, is viewed from the front. The optical axis of the liquid crystal molecule is in parallel with the absorption axis of the polarizer. Double refraction of the liquid crystal molecule does not occur, and light does not leak.
FIGS. 9A and 9B are views illustrating the shape of a liquid crystal molecule when viewing the vertical alignment type-liquid crystal display from the side at an angle to the front of the liquid crystal display.
When the voltage is not applied, as shown in FIG. 9A, the axis of the liquid crystal molecule is parallel with the absorption axis of the analyzer, and therefore, light does not leak.
When the voltage is applied, as shown in FIG. 9B, the axis of the liquid crystal molecule is offset from the axis of the polarizer or the axis of the analyzer. Consequently, double refraction of the liquid crystal molecule occurs, and light leaks.
When the light leakage phenomenon is used, the display contrast is lowered in the horizontal direction. As a result, it is not possible to recognize what is displayed even when the display is seen from a horizontal angle. Consequently, it is possible to control the confidentiality of the display using this light leakage phenomenon.
FIG. 10 is a view illustrating the specific construction for controlling the confidentiality of the display, where a single pixel includes sub-pixels of red, green and blue (RGB) and a sub-pixel of white (W).
FIG. 11 is a view illustrating the arrangement of liquid crystal molecules of the respective sub-pixels shown in FIG. 10. As shown in FIG. 11, the alignment state of the liquid crystal molecules in the white sub-pixel is quite different from the alignment state of the liquid crystal molecules in the RGB sub-pixels. Specifically, the liquid crystal molecules are aligned upward and downward in the white sub-pixel.
Consequently, when the voltage is not applied to the white sub-pixel, the white sub-pixel does not contribute to the display, and therefore, a normal display can be realized.
When the voltage is applied to the white sub-pixel, on the other hand, a white display is performed at the front in the horizontal direction. As a result, the contrast of the display is lowered in the horizontal viewing angle direction, and therefore, it is difficult for other people to view the display.
FIG. 12 is a plan view illustrating an enlarged pixel of the conventional vertical alignment type liquid crystal display, and FIG. 13 is a sectional view illustrating the enlarged pixel of the conventional vertical alignment type liquid crystal display.
FIGS. 14A and 14B are views illustrating the operation of liquid crystal molecules with the application of a voltage in the conventional vertical alignment type liquid crystal display.
A (chevron) “<”-shaped common electrode for controlling the liquid crystal inclination direction is formed on a transparent electrode at a color filter (CF) side (see FIGS. 12 and 13).
When the voltage is not applied, the liquid crystal molecules are oriented in the vertical direction, as shown in FIG. 14A.
When the voltage is applied, the liquid crystal molecules are inclined in the a prescribed direction by the common electrode due to the effect of the inclined electric field, i.e., the direction perpendicular to the spreading direction of the common electrode, as shown in FIG. 14B.
As a result, the inclination of the liquid crystal in two directions corresponding to the “<” shape is possible, and a liquid crystal display having a good viewing angle is realized.
However, the conventional liquid crystal display has the following problems.
First, the conventional liquid crystal display is constructed such that a white sub-pixel is formed; however, it is necessary to form a white resin, and the driving operation of the white sub-pixel is different from the conventional art.
Second, the contrast is lowered in the horizontal orientation; however, the contrast is not lowered in the vertical orientation.